Fluorescent lamp operating life has heretofore been limited to the amount of electron emissive coating available on the fluorscent lamp electrodes. When the emissive coating has been completely used up during lamp operation the lamp is no longer able to start and is considered defective and must hence be discarded. The discovery that heavy emissive coating quantities can be applied to the fluorescent lamp electrodes by the use of a barium tantalate emissive coating having the formulation 5BaO. Ta.sub.2 O.sub.5 has therefore produced fluorescent lamps capable of operating lifetimes far in excess of lamps having the standard triple carbonate coatings of the prior art.
The barium tantalate having the formulation described within the copending application entitled "Emissive Coating for Electrodes" - E. R. Kern-4 assigned to the assignee of the instant invention has greatly extended the operating lamp life of fluorscent lamps but deters from the cosmetic appearance of the lamp since it causes the premature formation of the unsightly end discoloration that is characteristic of lamps that are burnt for long periods of time. The exact nature of the end discoloration on fluorescent lamps is not fully understood but is believed to comprise a combination of the barium metal deposited from the emissive coating and oxides of mercury (present in the lamp envelope) that accumulate and react on the inner lamp envelope surface in the vicinity of the electrodes after a few hundred hours of operation. The early formation of the unsightly end discoloration of the barium tantalate emissive coating can be somewhat deterred by the addition of a quantity of calcium carbonate to the barium tantalate coating suspension. The improvement in the reduction of end discoloration with the addition of the calcium carbonate although helpful is not usually sufficient to bring the lamps containing the barium tantalate emissive coating within acceptable fluorescent lamp quality standards. The purpose of this disclosure therefore is to describe an improved emissive tantalate compound that has the aforementioned properties of extended fluorescent lamp life but improves the fluorescent lamp quality by eliminating the early formation of lamp end discoloration.